The invention relates to a heterojunction transistors of the field effect transistor category or of the bipolar transistor category, using III-V semiconductor alloys.
III-V semiconductors, of which the best known is gallium arsenide GaAs, constitute a vast family making use of one or more elements from column III and one or more elements from column V of the periodic table. Thus there exist:
binary compounds such as GaAs or InP;
ternary compounds such as AlGaAs or GaInAs or GaInP; and
quaternary compounds such as GaInAsP or AlGaSbAs.
In ternary or quaternary alloys, the III elements substitute for one another and likewise the V elements substitute for one another so that the III elements as a whole and the V elements as a whole are of equal composition: for example AlxGa1xe2x88x92xAs or GaxIn1xe2x88x92xAsyP1xe2x88x92y. To avoid overburdening the text, the simpler notation AlGaAs or GaInAsP is adopted below. Wherever necessary, the concentrations of the various elements are specified. Furthermore, the various elements making up an alloy are given in the standardized order (from the most electropositive to the least electropositive), and not in order of decreasing concentration.
The advantage of such ternary or quaternary alloys or indeed quinary alloys when making electronic components stems from the fact that substituting one III element for another or one V element for another modifies the electronic properties of the alloy, e.g. the effective mass of electrons or holes, or indeed the width of the forbidden band. These modifications are taken advantage of when making heterojunctions, i.e. junctions between two materials of different kinds, e.g. AlGaAs/GaAs or GaInAsP/GaAs, etc.
Semiconductor lasers made of AlGaAs/GaAs or GaInAsP/AlGaInP or InGaAs/GaAs/GaInP are well known examples of using heterojunctions to make lasers that emit at the following wavelengths respectively 870 nanometers (nm), 670 nm, and 980 nm.
So-called high electron mobility field effect transistors (HEMTs) making use of AlGaAs/GaAs or AlGaAs/GaInAs/GaAs heterojunctions are also well known for their performance which is better than that of conventional non-heterojunction GaAs transistors.
Heterojunction bipolar transistors (HBTs) of the AlGaAs/GaAs or GaInAsP/GaAs type are also known for their performance which is better than that of non-heterojunction GaAs bipolar transistors.
In spite of the flexibility with which III-V semiconductor alloys can be combined with one another in the form of a heterojunction, there nevertheless exists a limit on such combination.
Different materials can be associated with each other only if they have the same crystal lattice parameter or have lattice parameters that are very similar, so as to ensure that high mechanical stresses do not appear between the various materials. High stress can lead to dislocations appearing at the interface between the two materials, which dislocations then propagate throughout one of the two materials, thereby degrading the quality of such materials and consequently the quality of the electronic component. When the mismatch between the lattice parameters of the two parameters is small, it is possible to grow a heterojunction by epitaxy without causing dislocations to appear at the interface since the mechanical stress generated is small enough for the material to remain within its bounds for elastic deformation. It is known that elastic deformation is a function both of the difference between the crystal lattice parameters and of the thickness of the stressed layer. Below a certain thickness known as the xe2x80x9ccriticalxe2x80x9d thickness, the layer remains in a state of elastic deformation, i.e. free from dislocations, and above the critical thickness the stresses are relaxed by the appearance of dislocations.
Many electronic components make use of this possibility to achieve elastic stress in order to provide heterojunctions having electronic characteristics of interest. For example, AlGaAs/GaInAs heterojunctions are made in which the indium content can be as much as 25%, thereby creating a parameter mismatch approaching 2% and thus imposing a critical thickness of about 10 nm.
This stressed GaInAs material presents advantages over non-stressed GaAs material when making HEMTs: electron mobility is improved; the conduction band discontinuity at the heterojunction interface is advantageously greater. Such HEMTs using the narrow forbidden band material in the elastic stress state are referred as xe2x80x9cpseudomorphicxe2x80x9d HEMTs. Pseudomorphic HEMTs are in very widespread use for low noise amplification and for power amplification.
HBTs make little use of this pseudomorphic state because the base of an HBT is generally about 100 nm thick, and thus thicker than the critical thickness for lattice mismatch materials that are liable to present electronic characteristics of interest. Thus, no pseudomorphic HBTs are known as industrial products.
Furthermore, because heterojunctions are generally grown epitaxially on a substrate, it is the substrate which determines the lattice parameter. For practical reasons, substrates are binary compounds, and the compound in most widespread use industrially is GaAs. This is followed by InP which is difficult to manufacture and therefore expensive and which is also fragile and brittle. In spite of this handicap, Inp is often used because it makes it possible to implement alloys such as GaInAs or GaInAsP with very high indium contents, up to as much as 60%. Such alloys present electronic properties of interest, such as a narrow forbidden band extending from 0.6 electron volts (eV) to 1 eV.
Thus, in the present state of the art, the HEMTs in most widespread use can be put into two categories. The first category uses a GaAs substrate and arsenides as the narrow forbidden band material: GaAs for non-pseudomorphic HEMTs and GaInAs for pseudomorphic HEMTs having an indium content up to about 25%. The second category uses an InP substrate and arsenides as the narrow forbidden band material: GaInAs with indium at 52% for non-pseudomorphic HEMTs and at up to about 65% for pseudomorphic HEMTs.
In the same manner and in the present state of the art, HBTs can be put into two categories: those made on a GaAs substrate with GaAs as the narrow forbidden band material, and those on an InP substrate with GaInAs (52% indium content) as the narrow forbidden band material. As recalled above, pseudomorphic HBTs have not been developed industrially.
Nevertheless, both in HEMTs and in HBTs, the use of arsenides as the narrow forbidden band material presents limitations.
In HEMTs, the limitations occur as follows: the main characteristic of HEMTs is the association of a material having a broad forbidden band, generally AlGaAs with a material having a narrow forbidden band, GaAs or GaInAs, thus making it possible to obtain a two-dimensional electron gas that accumulates in the narrow forbidden band material when the broad forbidden band material is doped. This transfer of electrons from the broad forbidden band material to the narrow forbidden band material is more effective with increasing conduction band discontinuity xcex94Ec. Having a two-dimensional electron gas of high density makes it possible for the drain current of the transistor to be greater, and thus for power amplification to be more efficient. Unfortunately, in the arsenide system with an AlGaAs/GaAs or an AlGaAs/GaInAs combination, the xcex94Ec discontinuity is limited. Firstly xcex94Ec, which increases with the aluminum content in AlGaAs, cannot exceed a certain value, since above 22% aluminum, troublesome defects known as xe2x80x9cDX centersxe2x80x9d appear in the AlGaAs, and above 40% the AlGaAs material presents an indirect forbidden band. Secondly, adding indium to GaAs also makes it possible to increase xcex94Ec, but as recalled above, in practice it is hardly possible to exceed 25%. As a result, in practice, the AlGaAs/GaInAs system has a xcex94Ec maximum of about 400 millielectron volts (meV). In the category of HEMTs on an InP substrate where the broad forbidden band material is AlInAs (53% indium) and the narrow forbidden band material is GaInAs (up to 65% indium), xcex94Ec does not exceed 600 meV.
In HBTs, the limitations occur as follows: one of the important characteristics of an HBT is the base-mitter voltage VBE that must be applied to its base relative to the emitter in order to ensure that the collector receives current. This ON voltage VBE depends strongly on the width of the forbidden band of the material from which the base is made. For example, if the base material is GaAs, then the ON voltage VBE is about 1 volt. However, one of the major applications of HBTs, in volume terms, is power amplification in mobile telephones. In this application, the power supply voltage is delivered by a battery and for reasons, amongst other things, of battery weight and compatibility between the power supply voltage and other electronic functions contained in a mobile telephone, power supply voltage has been dropping as semiconductor technology advances. In 1999, the lowest voltage in use in pocket-sized telephones is about 3 volts, such that an HBT with a GaAs base and a VBE voltage of 1 volt is entirely viable since there still remain two usable volts. However when the voltage drops towards 1.5 volts or less, a VBE voltage of 1 volt. is no longer acceptable. It is therefore necessary to reduce the VBE voltage, in other words to have an HBT made of a material whose forbidden bandwidth is as narrow as possible. One solution would be to use the second category of HBTs grown epitaxially on InP, since the base made of GaInAs (52% indium content) has a forbidden band of about 0.75 eV which could lead to an ON voltage VBE of about 0.3 volts. However, as mentioned above, an InP substrate is expensive and fragile. The cost of the component would be too high for an industrial application in which cost reduction is a major factor.
To mitigate those drawbacks, the present invention essentially proposes using heterojunction transistors in which the narrow forbidden band material is constituted by an alloy making use simultaneously of at least two V elements, arsenic and nitrogen.
III-V components which contain both arsenic and nitrogen have the feature of presenting a forbidden bandwidth that is narrower than those which do not contain them. In other words, substituting a small fraction of arsenic with the same fraction of nitrogen greatly reduces the size of the forbidden band. For example, the compound GaAsN with 4% nitrogen has a forbidden bandwidth of about 1 eV, whereas the forbidden bandwidth of GaAs is 1.42 eV. This property is unexpected since given that GaN is a material having a very broad forbidden band (3.4 eV), a rule of thumb that often applies to III-V compounds would lead to assuming that the GaAsN compound had a forbidden band of greater width than that of GaAs. This exceptional property was mentioned for the first time by M. Weyers et al. in Jpn. J. Appl. Phys., Vol. 31 (1992), 853, and theoretical calculations confirm those experimental results (S. Sakai et al., Jpn. J. Appl. Phys., Vol. 32 (1993), 4413).
The lattice parameter of GaAsN compounds obeys Vegard""s law, i.e. it varies linearly with nitrogen content. Thus, when a thin layer of GaAsN is grown epitaxially on a GaAs substrate, the thin layer is in a stressed state, given that the crystal lattice parameter for GaN (0.45 nm) is smaller than that of GaAs (0.565 nm). Work by Y. Qui et al. (Appl. Phys. Lett., 70 (24), 3242 (1997)) and by E.V.K. Rao et al. (Appl. Phys. Lett., 72 (12), 1409 (1998)) shows that the critical thickness for layers of GaAsN grown epitaxially on a GaAs substrate is large and can be as much as 100 nm for a nitrogen content of the order of a few percent. Such a thickness is entirely suitable for making HEMTs or HBTs.
Since the lattice parameter of GaAsN is smaller than that of GaAs, it is possible to add a chemical element to this ternary compound for the purpose of increasing its lattice parameter. Thus, M. Kondow et al. (Jpn. J. Appl. Phys., Vol. 35 (1996), 1273) propose adding indium to form a quaternary compound GaInAsN. According to those authors, the quaternary compound presents a large conduction band discontinuity xcex94Ec relative to GaAs which indicates that it would be advantageous for use in making lasers having high temperature stability.
More precisely, the invention provides a heterojunction transistor comprising III-V semiconductor materials with a material having a broad forbidden band and a material having a narrow forbidden band, the transistor being characterized in that: the narrow forbidden band material is an III-V compound containing gallium as one of its III elements and both arsenic and nitrogen as V elements, the nitrogen content being less than about 5%; and the narrow forbidden band material has at least a fourth III or V element; in such a manner that adding this fourth element makes it possible to adjust the width of the forbidden band, the conduction band discontinuity xcex94Ec, and the valance band discontinuity xcex94Ev of the heterojunction.
The large xcex94Ec value of the GaInAsN compound can advantageously be used for making HEMTs.
Under such circumstances, according to various advantageous subsidiary characteristics:
the fourth element is indium, substituted for gallium by a fraction of not more than about 10% so as to provide a forbidden band of about 0.8 eV, a conduction band discontinuity of about 500 meV, and a conduction band discontinuity of about 100 meV;
the broad forbidden band material is AlGaAs with a molar fraction of aluminum of about 25%, so as to obtain a xcex94Ec discontinuity of about 800 meV and a xcex94Ev discontinuity of about 300 meV, i.e. GaInP having an indium molar fraction of about 50%, so as to provide a xcex94Ec discontinuity of about 500 meV and a xcex94Ev discontinuity of about 500 meV;
the fourth element is indium at a molar fraction of about 40%, thus obtaining a narrow forbidden band material with a lattice that is strongly mismatched relative to the GaAs substrate, this narrow forbidden band material is grown epitaxially on the GaAs substrate without dislocations, and the broad forbidden band material is AlInAs with about 40% indium, so as to obtain a xcex94Ec discontinuity reaching about 1 eV; and
the transistor is a field effect transistor of the pseudomorphic HEMT type, in that the narrow forbidden band material GaInAsN possesses indium and nitrogen contents such that its lattice parameter is greater than that of the GaAs substrate and so that it is in an elastically stressed state; and the broad forbidden band material is AlGaAs, so as to obtain xcex94Ec and xcex94Ev discontinuities of about 900 meV.
The invention also applies to making heterojunction bipolar transistors in which the narrow forbidden band material forms the base of the transistor and the broad forbidden band material forms the emitter of the transistor.
The fourth embodiment can then be indium, substituted for gallium, at a fraction of no more than about 10% so as to obtain a forbidden band of about 0.8 eV to 1 eV, and the broad forbidden band material being GaInP with an indium content of about 50% so as to obtain a AEv discontinuity of about 450 meV.
The material forming the base of the transistor preferably presents a nitrogen content that is either graded, nitrogen content being zero at the interface of the emitter-base heterojunction and increasing towards the base, or else varies stepwise, with the nitrogen content being zero at the emitter-base interface.
Nevertheless, the low value of the valance band discontinuity xcex94Ev of the GaInAsN material compared with GaAs can constitute a handicap when making HBTs. The present invention thus proposes using antimony as the fourth element, substituting the arsenic by a fraction of not more than about 10%, so as to obtain a forbidden band of about 0.8 eV to 1 eV.
In this case, the broad forbidden band material can be GaAs so as to obtain a xcex94Ec discontinuity of about 50 meV and a xcex94Ev discontinuity of about 400 meV, or else it can be GaInP with an indium content of about 50%, so as to obtain a xcex94Ec, discontinuity of about 100 meV and a xcex94Ev discontinuity of about 800 meV.
The narrow forbidden band material can be a quinary GaInSbAsN compound.